Using Thunderbird forever, on my PC and also on my Android. On my PC sending messages stopped working "suddenly". (That means I did not change anything in Thunderbird. Windows related settings - who knows.) __________________ Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) smtp.ionos.de failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again. __________________ I have checked the SMTP server settings again, but it didn't solve the problem. I have changed my password of my e-mail account to anew one, reinstalled Thunderbird and recreated my account, but without anything changing. Receiving works, sending does not. Tried to install Outlook (sorry, just temporarily, I hope :) ) and there everything works just fine. Maybe some Windows Firewall setting? Didn't find anything that seems to prevent Thunderbird from sending thou. Do you have any idea?